where \(n\)  is an arbitrary light-like vector and \(n\cdot k_g\ne 0\) .
In contrast to the covariant gauges the separate pieces, \({\cal H}^{P,\phi _{\pi 2}^{EOM}}\)  and \({\cal H}^{P,qqg,C_F}\) ,
are not invariant with respect to axial gauges but their sum does not depend on \(n\) .
Thus, we can conclude that the twist-3 subprocess amplitudes are QCD gauge invariant.
The choice of gauge for the external gluon was discussed in [1]}.
